The present invention relates to an improvement in electrical resistance welders, and more particularly, to resistance welders wherein a deep opening, commonly referred to as a "deep throat", is desirable or necessary.
While electrical resistance welding has been a viable method of welding for years, refinement of the art is constantly being explored. As the workpiece becomes wider the throat of the welder is increased in depth to accommodate the workpiece. The term workpiece, is used hereinafter to connote plural pieces of metal which are to be welded together.
Resistance welders have become a mainstay of automobile construction. Large and small resistance welders are used to weld the body portions of wheeled vehicles of all descriptions. The welding of large body portions such as floor pans, fenders, roof, hood, doors, frames, etc., presents a problem in that it is impractical for the vehicle body to be maneuvered with respect to a stationary welder, requiring the welder to be taken to the work.
Portable resistance welding guns, hereinafter sometimes referred to as guns or gun welders, have been introduced for the purpose of solving the problem of welding large body portions. These guns are, for the most part, standard equipment designs and are obtainable through any of a number of suppliers such as Milco Manufacturing Company. These portable guns allow the welding electrodes to be taken to the workpiece, while the transformer and controls, which are still relatively bulky, are stationary and located at a remote position from the welding machine. The conductors used to operate the portable gun are extremely bulky due to the current capacity required and the cross section to carry it.
Whether a stationary or portable welder is contemplated, the majority of the current provided to the welder is expended in the transfer to the welding electrodes. The power, i.e., I.sup.2 R, required to weld the workpiece is normally quite small as compared to the total power requirement of the welder. For example, in a conventional resistance welding set-up, at most 10% of the total power actually is required to produce the weld.
The current drain, and subsequent power loss, in the transfer of welding current to the electrodes is a direct result of the inductance due to the throat of the welder. The inductance in this case is caused by the arms of the welder, carrying relatively high current, in close proximity to one another. The inductance within the throat has been a problem since resistance welders were first put into service. While the problem is not eliminated, it is minimized by using a non-deep throat welder. Of course, this solution is not applicable where a deep-throat is necessary.
Since early in the development of resistance welders, cubic transformers have been chosen as the standard. In a cubic transformer both the primary coil and the secondary coil are generally multiple wound coils. The secondary coil is designed so that the terminals are essentially adjacent to one another. This design is convenient for direct connection of welder arms or electrodes to the terminals. Linear transformers are designed so that the transformer, at least the secondary, extends linearly along a straight line without being doubled, coiled or otherwise folded upon itself. The result of this construction is that the secondary terminals are as far removed from one another as possible. Due to difficulty involved with incorporating linear type transformers into a welder, linear transformers have practically been ignored.
An approach to energy conservation is the use of high frequency transformers, such as, over 60 cps. The total power requirement is reduced by using a frequency above 60 cps, which permits the use of smaller transformers and conductors. Customarily utilized frequencies range from about 180 cps to about 4000 cps, and their use typically requires a generating source.
Size and weight are of critical importance in a portable resistance welder which typically does not exceed about 100 pounds. When high frequency is utilized in portable welders the transformer has been arranged to be physically located on the supported end of the welding gun. Through such an arrangement, in which high frequency and low current power is used, the conductors carrying power to the welding gun are decreased in size (cross section), as opposed to the size and weight of a gun using 60 cps.
For various reasons, vehicle manufacturers have been searching for expanded uses of aluminum. As is well known, substituting aluminum for steel requires a greater thickness of aluminum to obtain comparable strength. Thus, to weld an aluminum workpiece which provides the same strength as a steel workpiece requires the welding of thicker aluminum pieces, and demands from about 2.25 to about 3 times as much welding current as is required for the steel workpiece. This demand combined with the high current loss characteristics of the welding electrodes and conductors results in the total current demand being relatively much larger than with steel of comparable thicknesses. Because of the larger current requirement with aluminum, the problems of designing an acceptably efficient welder are magnified. This is particularly true of an efficient portable welder. If the throat of the portable gun becomes too deep, even high frequency and/or placement of the transformer directly at the back of the welding gun fails to provide an acceptable solution.
In the case of a stationary resistance welder, the throat depth is generally increased by making the electrode arms physically longer. As noted above, the major portion of total power is dissipated in the transfer of current to the electrodes before welding occurs. Obviously, as the electrode arms increase in length, the power consumption increases exponentially.
In the assembly of large items, most notedly vehicle bodies, multiple spot welding is often utilized. U.S. Pat. No. 1,103,041 discloses a resitance welder for use in multiple weld applications. The reference also discloses the use of multiple transformers. The stated reason for this construction is to obtain an increase in potential across the electrodes. The workpiece is not intended to be positioned for welding between the transformers as the transformers are not located within the arms of the welder.
U.S. Pat. No. 1,312,845 discusses current loss due to a buildup of current within the portion of workpiece in the throat. An attempt is made to minimize the loss of current by utilizing a partial air core transformer. U.S. Pat. No. 1,362,962 is an example of a stationary welder in which the arms supporting the electrodes have been extended to provide a deep throat. A cubic transformer is physically constructed around the lower arm of the welder.
U.S. Pat. No. 2,491,169 suggests an alternate construction of a deep throat stationary welder. The primary coil of the transformer in this case is one large continuous coil which is physically folded upon itself, that is one half of the coil is doubled over the other half of the coil, to allow access to the throat. The secondary of the transformer is the arms of the welder or current path for the electrodes. A great deal of discussion is included with regard to the care taken to position the doubled coil halves relative to one another so that inductance is minimized rather than increased due to the relative proximity of the halves. In addition, the reference requires half of the primary to use the welder arms as metal core, with the other half being an air core.
One example of a portable welding gun is illustrated in U.S. Pat. No. 2,471,881. Both high frequency current (180 cps) and locating the transformer physically immediately adjacent to the welding gun are discussed in this reference. The construction of this welder is particularly directed to a non-deep throat welder.
Another portable welder is shown in U.S. Pat. No. 2,749,417. In the reference, an extremely small portion of the welding arms are utilized as the core of the transformer. The stated purpose in using the electrodes as the core is to reduce the overall size of the welding gun. As in the prior reference, the design chosen results in a non-deep throat welder.